Image sensors, i.e. photosensitive devices, are apparatus that convert optical images into electrical signals. Image sensors have been widely used in digital cameras and other optical electronic devices. According to the electronic components, image sensors are categorized into two types: Charge Coupled device (CCD) image sensors and Complementary Metal-Oxide semiconductor (CMOS) image sensors (CIS). Comparing with the CCD image sensors, the CMOS image sensors have lower production cost, lower power consumption; and higher integration level, etc. Thus, the CMOS image sensors have a rapid development on the aspects of wide dynamic range and low illumination, etc.
CMOS image sensors are a typical type of solid state image sensors. A CMOS image sensor often includes a pixel unit array, a row driver, a column driver, a logic timer control, an AD convertor, a data bus output terminal, and a control terminal, etc. The operation of the CMOS image sensor often includes reset, optical-electrical conversion, integration, and data reading, etc. Among the components of the CMOS image sensor, the pixel units are among the key components. As the basic optical sensing units, the pixel units determine the image quality of the CMOS image sensor.
According to the pixel structure, the CMOS image sensors are categorized into passive pixel sensors (PPS) and active pixel sensors (APS). According to the integration level, the CMOS image sensors are categorized into three-transistor active pixel sensors (3T-APS), pined photodiode four-transistor active pixel sensors (4T-APS); and pined photodiode five-transistor active pixel sensors (5T-APS), among which the 4T-APS are the most popular ones in the market. According to the incident direction of the light to the optical sensing area, the CMOS image sensors are categorized into Front-side-illumination (FSI) image sensors and Back-side-illumination (BSI) image sensors.
Currently, there are certain issues in the design of the pixel units of the high-speed CMOS image sensors. First, because the exposure time is relative short, a CMOS image sensor needs a large area photodiode (PD) to enhance the sensitivity. The large area PD causes the lateral electric field of the PD region to be weak. Thus, the charges of the signal can only be transported by the diffusion of carriers; and the transport efficiency is low. Second, the image lag phenomenon severely affects the image quality of the CMOS image sensors. How to avoid the image lag is a big challenge in the design of the high-speed CMOS image sensors. Third, the full well capacity (FWC) is an important parameter affecting the performance of the CMOS image sensors, and studies were conducted on how to increase the FWC so as to increase the dynamic range and the signal-to-noise ratio as another approach to enhance the image performance of the CMOS image sensors.
The disclosed device structures and methods are directed to solve one or more problems set forth above and other problems in the art.